Method and apparatus for time synchronized handover

ABSTRACT

Method and apparatus, performed autonomously in a user equipment (UE), for autonomous handover to a target cell of a radio access network (RAN). The method comprises determining, by the UE, an identity of the target cell; receiving, by the UE, a precise time reference broadcast by the target cell, the precise time reference indicating an elapsed time from a predetermined epoch; deriving, by the UE, an uplink timing adjustment based at least in part on the precise time reference, the uplink timing adjustment establishing uplink communication synchronization with the target cell; and transmitting, by the UE, an uplink signal to the target cell in accordance with the derived uplink timing adjustment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/664,442, filed Oct. 25, 2019, which claims the benefit of priority toU.S. Provisional Application Ser. No. 62/754,109 filed Nov. 1, 2018, thecontents of each of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention pertains to the field of packet-based datacommunications in a wireless network and in particular to a method andapparatus for time synchronized handover.

BACKGROUND

A radio access network (RAN) node in a 5^(th) generation (5G) system isconnected to a core network (CN) control plane entity through aninterface called NG-C (or N2) and to a CN user plane entity through aninterface called NG-U (or N3). The CN control plane entity is alsoconnected to user equipment (UE) through an interface called N1; the N1control plane messages are conveyed through the RAN node as non-accessstratum (NAS) signalling.

A RAN node may also be connected to user equipment (UE) via a radio linkinterface (Uu) that comprises several entities associated with the radiolink protocol stack: a physical layer (PHY) entity, a medium accesscontrol (MAC) entity, a radio link control (RLC) entity, a packet dataconvergence protocol (PDCP) entity, a service data adaptation protocol(SDAP) entity, and a radio resource control (RRC) entity.

A RAN node may be connected to other RAN nodes via an interface calledXn that includes both a control plane component (Xn-C) and a user planecomponent (Xn-U). In an LTE system, similar interfaces exist: a RAN nodeis connected to a CN through an S1 interface and to other RAN nodesthrough an X2 interface.

A conventional random access procedure in a 3^(rd) generationpartnership project (3GPP) long-term evolution (LTE) and new radio (NR)compliant system begins with two messages:

-   -   Msg1 is a signal transmitted in a physical random access channel        (PRACH) from a UE to a RAN node. The PRACH is designed to allow        simultaneous uplink transmissions from multiple UEs to be        detected by the RAN node even if those uplink transmissions are        not time-aligned at the RAN node receiver.    -   Msg2 is a control plane message transmitted in a physical        downlink shared channel (PDSCH) from the RAN node to the UE. The        control message—a random access response (RAR) message—includes        two information elements:        -   a cell radio network temporary identifier (C-RNTI) assigned            to the UE that is used to identify the UE in subsequent            control plane messages transmitted within a serving cell;            this includes downlink control information (DCI) transmitted            over a physical downlink control channel (PDCCH); and        -   a timing advancement (TA) that the UE should apply to            subsequent uplink transmissions to ensure they are aligned            to an uplink transmission slot boundary when received by the            RAN node. The timing advancement compensates for different            signal propagation delays associated with the location of            different UEs with respect to the RAN node receiver.

Once timing advancement has been applied, uplink transmissions from theUE are scheduled in a physical uplink shared channel (PUSCH) that ismore spectrally efficient than PRACH.

In 3GPP LTE and NR, a conventional handover from a source cell to atarget cell involves the following steps:

-   -   step 1. measurement configuration and reporting. In this step,        the source (i.e. current serving) RAN node provides a UE with        information to allow the UE to perform measurements on the        signal quality of downlink transmissions from neighbouring        cells. The UE then periodically reports those measurements to        the source RAN node.    -   step 2. target cell selection by the RAN. When the source RAN        node determines that signal quality in the current serving cell        has fallen below a predetermined threshold, the source RAN node        selects a target cell for the UE based on the previously        reported measurements.    -   step 3. handover preparation in the target RAN node. The source        RAN node prepares the target RAN node for handover through        signalling over an intra-RAN network. The source RAN node        provides the target with information related to the UE        configuration in the current serving cell and the target RAN        node provides the source with information related to the UE        configuration in the target cell. The information provided by        the target RAN node may include a C-RNTI and a dedicated random        access preamble for use in the target cell.    -   step 4. handover preparation of the UE. The source RAN node        instructs the UE to handover to the network-selected target cell        using the configuration provided by the target RAN node.    -   step 5. handover execution. The UE performs a random access in        the network-selected target cell using the designated C-RNTI and        preamble.    -   step 6. handover completion. When the target RAN node receives        an uplink transmission from the UE using the designated C-RNTI        and preamble, the target RAN node indicates to the source RAN        node that handover has been successfully completed.

If the UE does not initiate an uplink transmission using the designatedC-RNTI and preamble within a prescribed time period, handover is deemedto have failed and the UE may initiate radio link failure recoveryprocedures.

In 3GPP LTE and NR, a cell radio network temporary identifier (C-RNTI)is used to indicate the target UE (or group of UEs) in downlink controlinformation (DCI) transmitted by a RAN node. A DCI provides theindicated target UE (or group of UEs) with a grant of radio resourcesfor an uplink or downlink transmission. Rather than explicitlytransmitting the C-RNTI in each DCI, the target UE (or group of UEs) isidentified by using the C-RNTI to scramble the 24-bit cyclic redundancycheck (CRC) that is attached to a DCI and used for detectingtransmission errors in the DCI.

A conventional C-RNTI is a 16-bit value that is valid only within thecell currently providing a radio link connection to the UE. When aC-RNTI is used to scramble the CRC, the 16-bit C-RNTI is extended to24-bits with the most significant 8 bits (1≤j≤8) set to zero andS _(i) =C _(i) ⊕R _(i)where:

i (1≤i≤24) is a bit number with i=1 representing the most significantbit,

C_(i) is bit i of the (unscrambled) CRC,

R_(i) is bit i of the 24-bit zero-extended C-RNTI,

S_(i) is bit i of the scrambled CRC, and

⊕ is the xor (modulo-2 addition) operation.

In 3GPP LTE and NR, a precise time reference is broadcast over a radiolink in a system information block (SIB) to one or more UEs. The precisetime reference indicates the absolute time at which the last symbol of adesignated radio frame or sub-frame is transmitted. The granularity ofthe precise time reference is typically on the order of nanoseconds ormicroseconds. The precise time reference indicates the elapsed time froma predetermined epoch such as 1980-01-06 00:00:00 hours. The absolutetime is traceable to a master clock that distributes precise time tomultiple nodes within a RAN; the granularity of this time reference istypically on the order of nanoseconds. With knowledge of the timingadjustment (TA) applicable in a cell, a UE can synchronise its localclock with the precise time reference provided by the RAN.

This background information is provided to reveal information believedby the applicant to be of possible relevance to the present invention.No admission is necessarily intended, nor should be construed, that anyof the preceding information constitutes prior art against the presentinvention.

SUMMARY

An object of embodiments of the present invention is to provide a methodand apparatus for time synchronized handover in a wireless communicationsystem.

Embodiments herein provide, in one aspect, a method, performed in a userequipment (UE), for autonomous handover to a target cell of a radioaccess network (RAN) that circumvents the conventional measurementreporting and random access procedures. The method includes receiving,by the UE, a configuration for autonomous operation within a group ofcells of the RAN. The method further includes determining, by the UE,the target cell from the group of cells. The method further includesderiving, by the UE, an uplink timing adjustment that establishes uplinkcommunication synchronization with the target cell. The method furtherincludes transmitting, by the UE, a protocol data unit (PDU) to thetarget cell in accordance with the derived uplink timing adjustment.

In some aspects, provided is a method for autonomous uplinksynchronization with a target cell in a radio access network (RAN). Themethod comprises receiving, at the UE, a first precise time referencefrom a first cell of the RAN, the first precise time referenceindicating a first elapsed time from a predetermined epoch; determininga first uplink timing adjustment for use with the first cell, the firstuplink timing adjustment for establishing uplink synchronization withthe first cell; receiving, at the UE, a second precise time referencefrom the target cell of the RAN, the second precise time referenceindicating a second elapsed time from the predetermined epoch; andderiving, by the UE, a second uplink timing adjustment that establishesuplink synchronization with the target cell, the second uplink timingadjustment derived from the first and second precise time references andthe first uplink timing adjustment.

In some aspects, the first uplink timing adjustment is determined by theUE based at least in part on a timing adjustment message transmitted tothe UE from the first cell.

In some aspects, deriving the second uplink timing adjustment comprisessynchronizing a local UE clock to the first precise time reference usingthe first uplink timing adjustment; and determining the second uplinktiming adjustment based on the difference between the local UE clock andthe second precise time reference.

In some aspects, the method further comprises performing an uplink datatransmission from the UE to the target cell with the uplink datatransmission advanced according to the second uplink timing adjustment.

In some aspects, the uplink data transmission includes an identifierassigned to the UE for use in the target cell.

In another aspect, the identifier assigned to the UE is used to identifythe UE in subsequent messages transmitted by the target cell.

In some aspects, the subsequent messages include downlink controlinformation (DCI) transmitted over a physical downlink control channel(PDCCH).

In one aspect, at least one of the first and second precise timereferences is obtained from at least one of a broadcast in a systeminformation block (SIB), and provided on-demand to the UE through aradio resource control (RRC) message.

In yet another aspect, provided is a method for handover to a targetcell of a radio access network (RAN). The method comprises determining,by the UE, an identity of the target cell; receiving, by the UE, aprecise time reference broadcast by the target cell, the precise timereference indicating an elapsed time from a predetermined epoch;deriving, by the UE, an uplink timing adjustment based at least in parton the precise time reference, the uplink timing adjustment establishinguplink communication synchronization with the target cell; andtransmitting, by the UE, an uplink signal to the target cell inaccordance with the derived uplink timing adjustment.

In one aspect, the method further comprises deriving the uplink timingadjustment based at least in part on a timing adjustment messagetransmitted to the UE from a second cell.

In one embodiment, the UE determines the identity of the target cellbased on downlink signal measurements performed by the UE.

In another embodiment, the downlink signal measurements include adownlink signal quality measurement.

In some embodiments, the method further comprises determining, by theUE, that the downlink signal quality measurement has dropped below apreestablished signal quality threshold and selecting, by the UE, a newtarget serving cell.

In one aspect, the precise time reference is broadcast in a systeminformation block (SIB). In another aspect, a user equipment (UE) isprovided. The UE comprises a radio network interface for receiving andtransmitting protocol data units (PDUs), a processor and a memory devicestoring instructions. The instructions, when executed by the processor,cause the UE to receive a configuration for autonomous operation withina group of cells of a radio access network (RAN). The instructionsfurther cause the UE to determine a target cell from the group of cells.The instructions further cause the UE to derive an uplink timingadjustment that establishes uplink communication synchronization withthe target cell. The instructions further cause the UE to transmit aprotocol data unit (PDU) to the target cell in accordance with thederived uplink timing adjustment.

Further provided is a user equipment (UE) comprising a radio networkinterface for receiving and transmitting protocol data units (PDUs), aprocessor and a memory storing instructions. The instructions, whenexecuted by the processor, cause the UE to receive a first precise timereference from a first cell of a radio access network (RAN). Theinstructions further cause the UE to determine a first uplink timingadjustment for use with the first cell, the first uplink timingadjustment for establishing uplink synchronization with the first cell.The instructions further cause the UE to receive a second precise timereference from a second cell of the RAN. The instructions further causethe UE to derive a second uplink timing adjustment that establishesuplink synchronization with the second cell, the second uplink timingadjustment derived from the first and second precise time references andthe first uplink timing adjustment.

Also provided is a user equipment (UE) comprising a radio networkinterface for receiving and transmitting signals over a radio link; aprocessor; a local UE clock; and a memory storing instructions that whenexecuted by the processor cause the UE to receive, using the radionetwork interface, a first precise time reference from a first cell of aradio access network (RAN). the first precise time reference indicatinga first elapsed time from a predetermined epoch; determine a firstuplink timing adjustment for use with the first cell, the first uplinktiming adjustment for establishing uplink synchronization with the firstcell; receive, using the radio network interface, a second precise timereference from a target cell of the RAN, the second precise timereference indicating a second elapsed time from the predetermined epoch;and derive a second uplink timing adjustment that establishes uplinksynchronization with the target cell, the second uplink timingadjustment derived from the first and second precise time references andthe first uplink timing adjustment.

BRIEF DESCRIPTION OF THE FIGURES

Further features and advantages of the present invention will beapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 illustrates, in one embodiment, transmission of a precise timereference in a radio access network (RAN).

FIG. 2 illustrates, in one embodiment, a time synchronised handoverprocedure.

FIG. 3 illustrates, in one embodiment, a network model of a RAN mobilitygroup (RMG).

FIG. 4 illustrates, in one embodiment, a scheme of determining a timingadjustment using a precise time reference.

FIG. 5 illustrates, in one embodiment, an autonomous time synchronisedhandover with centralised user equipment (UE) context.

FIG. 6 illustrates, in one embodiment, an autonomous, time synchronisedhandover with distributed UE context.

FIG. 7 illustrates, in one embodiment, a scheme of operation in deliveryof downlink data via serving cell tracking.

FIG. 8 illustrates, in one embodiment, a scheme of operation in deliveryof downlink data via RMG flooding.

FIG. 9 illustrates, in one embodiment, a scheme of handover betweenunsynchronised network cells in a time synchronised handover thatcircumvents a conventional random access procedure.

FIG. 10 illustrates, in one embodiment, a structure of an electronicdevice.

FIG. 11 illustrates, in one embodiment, a method of operation performedin a user equipment (UE), for autonomous handover to a target cell of aradio access network (RAN).

FIG. 12 illustrates, in one embodiment, a method of operation performedin a user equipment (UE), for autonomous synchronisation within a radioaccess network (RAN).

FIG. 13 illustrates, in one embodiment, a method of operation performedin a user equipment (UE), for autonomous handover from a source cell toa target cell of a radio access network (RAN).

FIG. 14 illustrates, in one embodiment, a method of operation performedin a user equipment (UE), for autonomous synchronisation within a radioaccess network (RAN).

Throughout the appended drawings, like features are identified by likereference numerals.

DETAILED DESCRIPTION

Embodiments of the present invention provide advantages by way ofautonomous, time synchronized handover performed by a UE that avoidssignalling overheads associated with conventional handover (e.g.measurement configuration, measurement reports, time-critical handoverpreparation of target RAN nodes, etc). Autonomous uplink synchronisationby a UE avoids the latencies and overheads of a random access procedureprior to transmission of uplink data. Use of a RAN mobility group RNTIavoids latencies and overheads in a target RAN node associated withconventional use of a random access procedure for assignment of an RNTIfor DCI signalling. A precise time reference broadcast over the air bycells in a RAN can be used by a UE to autonomously compute the timingadjustment required for operation in a new target cell.

In particular, solutions presented in the disclosure herein address thefollowing problems with current wireless system implementations.

Embodiments of the present invention relate to addressing problems withUE handover. Conventionally, handover is preceded by a measurement andmeasurement reporting phase that incurs signalling overheads and delaysassociated with the measurement period. When executing a handover andmoving from an old serving cell to a new serving cell, a UEconventionally follows a 4-step random access procedure which increaseslatency and incurs signalling overheads. A RACH-less handover has beenproposed as a means to reduce latency. However, a RACH-less handover canonly be performed between two cells where radio link transmissions inthe two cells are synchronised—e.g. the start of a frame or sub-frameboundary in the old and new serving cells must occur at the same time.In contrast, solutions described in the disclosure herein avoid themeasurement reporting phase and bypass the 4-step random accessprocedure without requiring synchronisation between the old and newserving cells, thus providing a reduction in latency and signallingoverheads.

Embodiments of the present invention relate to addressing problems withhandover. In contrast to conventional handover procedures where the RANselects a new serving cell, conditional handover has been proposed as ameans to reduce handover failures by allowing a UE to select its newserving cell based on its own measurements, thus avoiding delaysassociated with measurement reporting. Selection of the new serving cellby the UE is based on a set of target cells and radio signal qualitythresholds provided to the UE by the RAN.

Once the new serving cell has been selected, the UE follows aconventional random access procedure to move from the old to the newserving cell. Target RAN nodes for a conditional handover must still beprepared by the source RAN node prior to handover, therefore resourcesmust be reserved for the UE in each of the target cells until the UEselects and re-connects to the new serving cell. While they arereserved, the resources in each of the target cells may not be availablefor use by other UEs served by the target cell. In contrast, thesolutions described in this disclosure allow the UE to autonomouslyselect a new serving cell but do not require resources to be reservedspecifically for the UE in each of the potentially new serving cells.

Embodiments of the present invention provide for shortened orreduced-overhead uplink data transmission. Conventionally, before a UEcan transit uplink user plane data in a new serving cell, the servingRAN node must provide a UE with information such as timing adjustment(TA) and C-RNTI that is conveyed through control plane signalling thatincreases latency and overheads. In some instances, such as machine-typecommunications (MTC) where the user plane data consists of tens ofbytes, the control plane traffic may be greater than the user planetraffic. In contrast, the solutions described in this disclosureeliminate some of the control plane signalling that normally precedes anuplink user plane transmission in a new serving cell.

Embodiments of the present invention relate to addressing problemsrelated to unsynchronised uplink transmissions. Theoretically, uplinktransmissions may be asynchronous where the signals transmitted frommultiple UEs are not time-aligned at the RAN receiver. Howeverasynchronous UE transmissions reduce spectral efficiency and increasethe complexity of the RAN receiver, especially when asynchronous uplinktransmissions are used in contention-based uplink channels or inmultiple access uplink channels such as those using non-orthogonalmultiple access (NOMA). Use of a contention-based or multiple accessuplink channel is preferred for low latency communications withunpredictable traffic arrival times in order to avoid delays associatedwith requesting and subsequently scheduling an uplink transmission ondedicated (contention-free) radio resources. However the potentialadvantage of these channels is reduced if the UE must first synchroniseits uplink transmissions with the RAN, a process that may itself incuradditional latency and signalling overheads. In contrast, the solutionsdescribed in this disclosure provide a reduction in latency andsignalling overheads required for a UE to synchronise its uplinktransmissions with a RAN receiver.

This disclosure describes mechanisms to enable a UE for autonomoushandover through the use of time synchronisation to minimise overheadsassociated with connection to a new serving cell and with the subsequenttransmission of an uplink data packet by the UE. A UE may be a wirelessdevice, mobile station, fixed station, mobile node, human-typecommunications device, machine-type communications device, or other typeof untethered device.

“Autonomous handover” as used herein means that the UE may select atarget cell for handover based on its own local measurements rather thanbeing directed to a new serving cell by the RAN—i.e. selection of a newserving cell is performed by a UE rather than by the network. Targetcell selection is similar to the procedure used by a UE operating in aninactive state, however procedures for PDU session management aresimilar to those used by a UE operating in an active (e.g.RRC_CONNECTED) or inactive (e.g. RRC_INACTIVE) state.

To reduce latency and signalling overheads, a UE may avoid conventionalrandom access procedures when entering a new serving cell byautonomously determining the uplink timing adjustment required in thenew serving cell based on a precise time reference broadcast by eachcell.

In relation to the above, it is noted that transmissions by differentcells in a communication system are potentially not frame-aligned, sothat the precise time references transmitted by different cells may notbe the same. For example, as illustrated in FIG. 1 , the transmission ofa reference frame from cell A is not time-aligned with the transmissionof a reference frame from cell B; therefore the precise time referencetransmitted by cell A (RT_(A)) is different from the precise timereference transmitted by cell B (RT_(B))—e.g. RT_(B)=RT_(A)+ΔRT. Inaddition, the signal propagation delay to the UE from difference cellsmay be different. For example, as illustrated in FIG. 1 , the signalpropagation delay from cell A (Δt_(A)) is different from the signalpropagation delay from cell B (Δt_(B)).

With reference now to FIG. 2 , the operations in an autonomous, timesynchronised handover, according to an embodiment of the presentinvention, are described as follows.

At operation 201, preparation of a RAN mobility group is performed. ARAN mobility group (RMG) comprises a set of cells subtending one or moreRAN nodes that have been configured for autonomous UE handover.Preparation of a RAN mobility group may be performed by a managementplane entity through an OAM procedure where an RMG may be pre-configuredwithin a public land mobile network (PLMN) or may be performed by a RANnode that dynamically configures the RAN mobility group through controlplane signalling between RAN nodes. A RAN node may be an access point,base station, Node B, evolved Node B (eNB), next generation Node B (NRgNB and LTE ng-eNB), centralised unit (CU), distributed unit (DU) orother form of radio access controller.

At operation 202, preparation of a UE for mobility within an RMG isperformed. A source RAN node prepares a UE for mobility within an RMG byconfiguring the UE with an RMG radio network temporary identifier (RMGRNTI) and a set of shared RMG radio resources. The RMG RNTI identifiesthe UE within any of the cells of the configured RMG and the shared RMGradio resources are available for use by one or more UEs within any ofthe cells of the configured RMG.

At operation 203, a UE performs autonomous UE selection of a new servingcell. When the UE has uplink data queued for transmission, it executes acell selection procedure based, for example, on signal measurements madeautonomously by the UE and selects a suitable target serving cell thatis also one of its configured RMG cells. For example, the UE may selectthe cell with the strongest received signal. The target serving cell isselected autonomously by the UE without an exchange of control planemessages with the RAN.

At operation 204, the UE performs synchronous re-entry at the targetserving cell. The UE determines the timing adjustment required tosynchronise its uplink transmissions with the target serving cell byusing a precise time reference broadcast by the target serving cell. Thetiming adjustment is determined autonomously by the UE without anexchange of control plane messages with the RAN.

At operation 205, the UE performs transmission of uplink data in the newserving cell. Using the computed timing adjustment, the UE bypasses theconventional random access procedure and transmits an uplink data PDU tothe new serving cell using the previously assigned RMG RNTI and sharedRMG radio resources.

FIG. 3 illustrates, in one embodiment, a network model of a RAN mobilitygroup (RMG) 300. A RAN mobility group (RMG) comprises a set of cellswithin a public land mobile network (PLMN) that have been configured forautonomous handover by UE to a target cell.

A source RAN node 330 maintains connections to the core network (CN)control plane function (CPF) 340 for the UE (e.g. via a 5G NG interfaceor a 4G S1 interface). The source RAN node 330 maintains, or has accessto, the current configuration and other contextual information 332associated with the UE 310. The RAN node 330 is typically the lastserving RAN node for the UE 310.

One or more target nodes 360 denote the RAN nodes with one or moresubtending cells included in the set of cells that comprise the RMG 300.When a UE 310 autonomously selects a serving cell subtending a targetRAN node 360 that is different from the source RAN node 330, that targetRAN node becomes the new serving RAN node 320. In contrast tonetwork-directed handover, RAN nodes within the RMG retain their role asa target RAN node 360 even when the UE completes an autonomous handoverto a new serving RAN node 320—i.e. a RAN node is removed from an RMGeither through a re-configuration of the RMG or, in some embodiments,through expiration of a timer.

The source RAN node 330 may be connected via an intra-RAN network to oneor more other RAN nodes 360 within the PLMN. The interface between RANnodes, called Rn 350 in FIG. 3 , may be similar to an X2 interface, toan Xn interface, or to a CU-DU interface.

In some embodiments, the lower layers of the radio link protocol stack(PHY, MAC and RLC) are handled by the new serving (target) RAN node 320for a UE 310 operating in autonomous mode while the upper layers of theprotocol stack (PDCP and SDAP) are handled by the last serving (source)RAN node 330. In these embodiments, the source RAN node 330 maintainsconnections to at least one CN CPF 340 and to at least one CN user planefunction (UPF) 345.

In some embodiments, all layers of the radio link protocol stack (PHY,MAC, RLC, PDCP and SDAP) are handled by the new serving (target) RANnode 320. In these embodiments, the new serving (target) node 320 mayalso maintain connections to at least one CN UPF 345.

An RMG 300 may be associated with a particular UE or may be associatedwith multiple UEs. When associated with multiple UEs, an RMG may bepre-configured within a public land mobile network (PLMN) (e.g. throughan operations administration and maintenance (OAM) procedure) or may bedynamically configured through control plane signalling between RANnodes. When associated with a particular UE, the RMG is typicallyconfigured dynamically by the source RAN node.

A cell subtending a particular RAN node may be associated with zero, oneor more RMGs. In some embodiments, a cell may be configured to broadcastone or more RMG identifiers in a system information block (SIB). A UE istypically associated with at most one RMG.

While operating autonomously within an RMG, a UE is assigned an RMGradio network temporary identifier (RNTI) by its source RAN node thatallows the UE contextual information and, optionally, the source RANnode to be identified by a target RAN node within the RMG. An RMG RNTImay be assigned for exclusive use by the UE or it may be shared bymultiple UEs.

While a conventional cell RNTI (C-RNTI) is a 16-bit value that is validonly within the cell serving the UE when the RNTI was assigned, an RMGRNTI is a 24-bit value that is valid in all cells of the RMG. To preventconflicts with cell-specific C-RNTI values, an RMG RNTI must have one ormore non-zero bits in the most significant 8 bits (i.e. in bits 1-8where bit 1 is the most significant bit).

When an RMG RNTI is used to scramble the CRC in a DCI message, the full24-bits of the RMG RNTI are used—i.e. an RMG RNTI is not zero-extendedlike a conventional C-RNTI when used to scramble a DCI CRC. Thescrambled CRC is computed using the conventional algorithm as describedabove:S _(i) =C _(i) ⊕R _(i)

An RMG RNTI may be an unstructured 24-bit value from a flat number spaceor it may be a structured value that encodes at least a source RAN nodeidentifier and a UE context identifier. In some embodiments, theassociation between an unstructured RMG RNTI and the source RAN node maybe explicitly signalled by the source RAN node to other RAN nodes in theRMG when the RMG RNTI is assigned to a UE. In other embodiments, a setof RMG RNTIs may be associated with a particular RAN node throughdynamic signalling between RAN nodes or through an OAM procedure.

In other embodiments, the association between an unstructured RMG RNTIand a UE context identifier may be explicitly signalled by the sourceRAN node to other RAN nodes in the RMG. In some embodiments, the UEcontext associated with the RMG RNTI may also be explicitly signalled bythe source RAN node to other RAN nodes in the RMG.

In some embodiments of a structured RMG RNTI, the source RAN nodeidentifier may comprise the n most significant bits of the RMG RNTI andthe UE context identifier may comprise the remaining (24−n) bits wherethe value of n may be pre-defined or configurable. In other embodiments,the boundary between source RAN node identifier and UE contextidentifier may be determined through encoding of the RMG RNTI. Forexample, the encoding format may be identified in the m most significantbits from which the value of n, encoding the source RAN node identifier,may be determined.

While operating autonomously within an RMG, a UE may be provided withthe configuration of an RMG uplink shared data channel (USDCH) that maybe used by the UE to transmit an uplink protocol data unit (PDU) withina new serving cell. The radio resources associated with the RMG USDCHare available to the UE in all cells of its configured RMG allowing a UEto transmit an uplink PDU without first receiving, via a DCI message, anexplicit allocation of radio resources from the serving cell.

Embodiments of an RMG USDCH may include a contention-based channel wheresimultaneous uplink transmissions using the RMG USDCH collidedestructively and prevent decoding of the transmissions by a RAN node. AUE intending to transmit a PDU using the RMG USDCH may use a collisionavoidance procedure such as listen-before-talk (LBT) to reduce thepossibility of a collision. If a collision is detected, a UE may use acollision resolution procedure such as randomised backoff intervalbefore attempting a retransmission of the PDU using the RMG USDCH.

Embodiments of an RMG USDCH may include a multiple-access channel wheresimultaneous uplink transmissions using the RMG USDCH may besuccessfully decoded by a RAN node. For example, a non-orthogonalmultiple access (NOMA) channel allows simultaneous uplink transmissionsfrom multiple UEs to be successfully decoded based on the demodulationreference signal and code book used by a UE for encoding its uplinktransmission.

One or more uplink shared data channels may be configured within thecells of an RMG. When a UE is configured for autonomous handover withinan RMG, the source RAN node may provide the UE with informationassociated with its assigned USDCH; this information may include one ormore of: time resource, frequency resource, transmit power level,demodulation reference signal, code book, preamble, modulation andcoding scheme.

Other parameters associated with an RMG USDCH may be dynamicallysignalled by a cell in the RMG using, for example, a system informationblock (SIB) or a DCI in a physical downlink control channel (PDCCH). Insome embodiments, the parameters signalled by a cell may designate apool of radio resources (e.g. preamble, demodulation reference signal,code book) from which a UE randomly selects the resource to be used forits transmission in the USDCH.

FIG. 4 illustrates, in one embodiment, a procedure 400 for determining atiming adjustment using a precise time reference. In order to bypass theconventional random access procedure, a UE can autonomously determinethe timing adjustment (TA) required for uplink transmission in a newserving cell (cell B) from the timing adjustment used in its currentsource cell (cell A) and from the precise time reference broadcast bythe source and target cells.

At operation 401, the UE receives a precise time reference from cell A(RT_(A)) and may receive a precise time reference from cell B (RT_(B)).As indicated earlier, cell transmissions may not be frame-aligned sothat the transmitted reference time transmitted by cell B may not be thesame; for example, RT_(B)=RT_(A)+

RT.

RT is thus defined as a difference between the two precise timereferences.

At operation 402, the UE is synchronised to cell A and has received anuplink timing adjustment (

t_(A)) from cell A to compensate for the signal propagation delay.

At operation 403, the UE is then able to synchronise the time of itslocal clock LT_(A) to the reference time from cell A (RT_(A)); that is,LT_(A)=RT_(A)+Δt_(A).

At operation 404, the UE subsequently receives a precise time referencefrom cell B (RT_(B)). At operation 405, the UE notes that the referencetime RT_(B) was received at time LT_(B) according to its local clock;that is, LT_(B)=RT_(B)+Δt_(B). LT_(A) and LT_(B) may be viewed as thetimes, according to the UE's local clock, at which the reference timesRT_(A) and RT_(A) are received, respectively.

At operation 406, the UE is then able to determine the timing adjustment(Δt_(B)) required to synchronise uplink communications with cell B; thatis, Δt_(B)=(LT_(B)−RT_(B)).

As will be readily understood from the above, and in more general terms,deriving the timing adjustment (Δt_(B)) can include synchronising alocal UE clock to the first precise time reference RT_(A) using thefirst uplink timing adjustment Δ_(A); and determining the second uplinktiming adjustment based on the difference between the local UE clock andthe second precise time reference RT_(B).

The UE can obtain the reference time from the source cell (e.g. RT_(A)from cell A) at any time prior to obtaining the reference time from thetarget cell (e.g. RT_(B) from cell B). Once obtained, the reference timefrom the source cell (e.g. RT_(A)) is deemed to be valid for a period oftime (the synchronisation period) where the potential drift of the localUE clock is less than some maximum value; in some embodiments, the clockdrift cannot exceed the cyclic prefix of an LTE or NR OFDM symbol. Ifnecessary, the UE should obtain an updated reference time and timingadjustment from the source cell before expiration of the synchronisationperiod.

FIG. 5 illustrates, in one embodiment, a procedure for UE autonomous,time synchronised handover where the UE context 332 is maintained onlyat the source (last serving) RAN node 330. In this procedure, the lowerlayers of the radio link protocol stack (PHY, MAC and RLC) are handledby the new serving (target) RAN node 320 while the upper layers of theprotocol stack (PDCP and SDAP) are handled by the last serving (source)RAN node 330.

At operation 501, the configuration for a RMG is distributed to all RANnodes with cells in the target RMG. The RMG configuration, which may bepre-configured through an OAM procedure or (as shown) dynamicallysignalled by the source RAN node 330, may include a USDCH configuration,an RMG RNTI used to identify one or more UEs within the RMG, and theidentity of the source RAN node 330 associated with the RMG RNTI.

At operation 502, to prepare the UE for autonomous mobility, the sourceRAN node 330 may configure the UE 310 with a set of cells and/or an RMGidentifier comprising the RMG where the UE may operate autonomously, aUSDCH configuration for use by the UE within the RMG, and an RMG RNTI tobe used by the UE 310 for identification within the RMG.

At operation 503, once configuration by the source RAN node 330 has beencompleted, the UE 310 may begin autonomous operation within the RMG. TheUE 310 may continue to operate autonomously within the RMG until itreceives a configuration from the RAN disabling autonomous operation.The UE 310 may be required to disable autonomous operation if it roamsoutside the coverage area of the RMG. In some embodiments, the UE 310may be required to disable autonomous operation after expiry of a timerconfigured by the RAN. The UE 310 may operate autonomously while in anactive state (such as RRC_CONNECTED) or while in a low-energy state(such as RRC_INACTIVE).

At operation 504, at a given point in time, the UE 310 has uplink dataqueued for transmission and begins cell selection to select a targetserving cell. If the selected cell is in the configured RMG, the UE 310may continue (at operation 505) with autonomous operation; otherwise,the UE 310 may be required to disable autonomous operation andre-connect to the selected cell (e.g. by using a random accessprocedure) to resume non-autonomous operation in a connected state (suchas RRC_CONNECTED).

The UE 310 may be configured by the RAN to give preference to a targetcell that is within the RMG over a cell that is not within the RMG, thatis, a cell that is not within the RMG will be selected only if no cellthat is within the RMG can be found with a suitable signal quality.

At operations 505-508, the UE derives an uplink transmission timingadjustment for the selected target cell (TA_t). At operation 505, the UE310 obtains downlink frame synchronisation with the target cell byacquiring synchronisation signals broadcast by the cell. In 3GPP LTE andNR, for example, this may be the primary and secondary synchronisationsignals broadcast in a synchronisation signal block (SSB).

Operation 506, where the UE 310 obtains a precise time reference fromthe source cell (RT_s), may be performed at any time prior to operation508; operation 506 may, for example, be performed prior to any ofoperations 505, 504, 503 or 502.

At operation 507, the UE 310 obtains a precise time reference from thetarget cell (RT_t) and, if it has not already done so (at operation506), from the source cell (RT_s). In 3GPP LTE and NR, for example, theprecise time reference may be broadcast in a system information block(SIB) or may be provided on-demand to a UE through a radio resourcecontrol (RRC) message.

At operation 508, using knowledge of the timing adjustment used in thesource cell (TA_s), the UE 310 computes the timing adjustment requiredin the target cell (TA_t) as described, for example, in operation 406.

At operation 509, using the USDCH configuration provided by the sourceRAN node 330 in operation 502, the UE waits for a transmissionopportunity and initiates an uplink transmission using the assignedUSDCH resources and with the uplink transmission advanced according tothe timing adjustment computed in operation 508.

At operation 510, the information included in the uplink USDCHtransmission comprises at least the RMG RNTI assigned to the UE. Ifthere are sufficient USDCH resources assigned, the UE 310 may alsoinclude a MAC buffer status report (BSR) and/or (the first segment of)an RLC data PDU containing the queued data and corresponding logicaldata channel identifier.

At operation 511, the target RAN node 320 detects the USDCH demodulationreference signal, decodes the uplink USDCH transmission and retrievesthe RMG RNTI transmitted by the UE 310.

At operation 512, the target RAN node 320 schedules an uplinktransmission for the UE 310 and signals the assigned uplink resources ina DCI message encoded with the RMG RNTI received from the UE 310 inoperation 511.

At operation 513, using the assigned uplink resources, the UE 310transmits (another segment of) an RLC data PDU containing the queueddata and corresponding logical data channel identifier. The UE 310 mayalso include a MAC buffer status report to indicate whether additionaluplink data is queued at the UE 310. This uplink transmission istypically scheduled in dedicated radio resources over a contention-freeuplink channel. Subsequent uplink and/or downlink transmissionsscheduled by the target RAN node 320 (e.g. for hybrid automatic repeatrequest (HARD) and RLC re-transmissions) continue to be signalled withDCI messages encoded using the RMG RNTI.

At operation 514, the target RAN node 320 associates the received RMGRNTI with the corresponding source RAN node 330 based either onpre-configured RMG information (operation 501) or on the encoding of astructured RMG RNTI.

At operation 515, the target RAN node 320 extracts the PDCP data PDUtransmitted in the RLC data PDU and forwards the PDCP data PDU to thesource RAN node 330 along with the RMG RNTI and logical data channelidentifier provided by the UE 310.

At operation 516, using the security configuration from the UE contextassociated with the RMG RNTI, the source RAN node 330 decrypts andvalidates the PDCP data PDU.

At operation 517, the source RAN node 330 forwards the resulting userplane data PDU to the CN user plane function (UPF) associated with thelogical data channel indicated by the UE 310.

FIG. 6 illustrates a procedure for UE autonomous, time synchronisedhandover where the UE context 332 is distributed by the source (lastserving) RAN node 330 to target RAN nodes 360 within the RMG. In thisprocedure, all layers of the radio link protocol stack (PHY, MAC, RLC,PDCP and SDAP) are handled by the new serving (target) RAN node 320using the stored UE context. In some embodiments, the new serving(target) RAN node 320 may be able to forward uplink PDUs directly to theCN UPF 345 rather than forwarding uplink PDUs indirectly through thesource RAN node 330.

At operation 601, as in operation 501 of FIG. 5 , the configuration foran RMG is distributed to all RAN nodes with cells in the target RMG. Inaddition, the RAN nodes of the target RMG also receive an indication ofthe RAN context that is associated with UEs authorised for autonomousoperation within the RMG. At operations 502-513, the UE 310 is thenprepared and begins autonomous handover as described in operations 502through 513 of FIG. 5 .

At operation 614, when the target RAN node 320 decodes the RMG RNTItransmitted by the UE 310, it identifies and retrieves the UE contextassociated with the RMG RNTI based on the pre-configured RMG information(operation 601).

At operation 615, the target RAN node 320 extracts the PDCP data PDUtransmitted in the RLC data PDU and, using the security configurationfrom the UE context associated with the RMG RNTI, decrypts and validatesthe PDCP data PDU.

At operation 616, if the UE context includes the identity of theassociated CN user plane function (UPF) associated with the logical datachannel indicated by the UE 310, the target RAN node 320 forwards theresulting user plane data PDU to the CN UPF 345. Otherwise the targetRAN node 320 forwards the resulting user plane data PDU to the sourceRAN node 330 associated with the RMG RNTI.

In order to deliver downlink PDUs to a UE operating in an autonomousmode, the source RAN node 330 may configure the UE 310 for serving celltracking to ensure that the current serving cell and serving (target)RAN node 320 is always known to the source RAN node 330. If the UE 310is not configured for serving cell tracking, a downlink PDU may beforwarded to all target RAN nodes within the RMG for attempted delivery.

The delivery procedure chosen by the RAN for a particular UE may involvea trade-off between the signalling overheads associated with servingcell tracking versus the signalling overheads associated with attempteddelivery in multiple RAN nodes. For example, serving cell tracking maybe more beneficial for a low-mobility UE that does not often change itsserving cell or for a UE that receives a high volume of unexpecteddownlink traffic; attempted delivery in multiple RAN nodes may be morebeneficial for a high-mobility UE that often changes its serving cell orfor a UE that receives a low volume of unexpected downlink traffic.

FIG. 7 illustrates, in one embodiment, a scheme of operation in deliveryof downlink data via serving cell tracking. When configured forautonomous operation within an RMG 300, a UE 310 may be instructed toselect a new serving cell, which may be associated with a new serving(target) RAN node 320, and to transmit a tracking report whenever thesignal quality measured by the UE 310 in its current serving cell dropsbelow a certain threshold. Downlink data destined for the UE 310 can beforwarded from the source RAN node 330 directly to the current serving(target) RAN node 320; the current serving (target) RAN node 320 canthen schedule the received data for delivery to the UE 310 at the nextdownlink transmission opportunity.

At operation 701, as in operation 501 of FIG. 5 , the configuration foran RMG is distributed to all RAN nodes with cells in the target RMG.

At operation 702, as in operation 502 of FIG. 5 , the UE 310 is preparedfor autonomous operation within the RMG. In addition, the source RANnode 330 may configure the UE 310 with a tracking configuration,indicating when the UE 310 should send a tracking report to the RAN,which may include a downlink signal quality threshold and a maximum timeinterval between tracking reports, and a discontinuous reception (DRX)schedule for use by the UE 310 within the RMG, indicating when the UE310 should monitor the physical downlink control channel (PDCCH) of itsserving cell for notifications of pending downlink data delivery.

At operation 703, once configuration by the source RAN node 330 has beencompleted, the UE 310 may begin autonomous operation within the RMG.

At operation 704, if the UE 310 determines that the downlink signalquality in its current serving cell has dropped below the configuredthreshold or that the maximum time interval between tracking reports hasbeen reached, the UE autonomously selects a new target serving cell asdescribed in operation 504 and derives an uplink transmission timingadjustment for that cell (TA_t) as described in operations 505 to 508 ofFIG. 5 .

At operation 705, the UE 310 constructs a control plane tracking report(e.g. a radio resource control (RRC) message) and, using the USDCHconfiguration provided by the source RAN node 330 in operation 702,waits for an uplink transmission opportunity.

At operation 706, the UE 310 initiates an uplink transmission using theassigned USDCH resources and with the uplink transmission advancedaccording to the timing adjustment computed in operation 704. The uplinktransmission, similar to operations 510 through 513 of FIG. 5 , includesthe RMG RNTI assigned to the UE and a PDCP data PDU containing thecontrol plane tracking report.

At operation 707, the target RAN node 320 associates the received RMGRNTI with the corresponding source RAN node 330 based on pre-configuredRMG information (operation 701).

At operation 708, the target RAN node 320 forwards the PDCP data PDU tothe source RAN node 330 along with the identity of the new serving cell,the RMG RNTI and logical data channel identifier associated with thesignalling radio bearer provided by the UE 310.

At operation 709, using the security configuration from the UE contextassociated with the RMG RNTI, the source RAN node 330 decrypts andvalidates the PDCP data PDU containing the tracking report. If valid,the source RAN node 330 updates its stored UE current location using thenew serving cell identifier provided by the target RAN node 320.

At operation 710, at some point in time, the source RAN node 330receives a downlink user data PDU destined for the UE 310 from the CNUPF 345.

At operation 711, using the security configuration associated with theUE context, the source RAN node 330 constructs and encrypts a PDCP dataPDU containing the downlink user data PDU.

At operation 712, using the stored UE current location, the source RANnode 330 forwards the PDCP data PDU to the current serving (target) RANnode 320 along with the RMG RNTI and the DRX configuration provided tothe UE 310 in operation 702.

At operation 713, the current serving (target) RAN node 320 uses the DRXconfiguration to schedule a transmission to the UE at the next downlinktransmission opportunity.

At operation 714, similarly, UE uses the DRX configuration provided inoperation 702 to monitor the downlink control channel (e.g. PDCCH) inthe current serving cell at the next downlink transmission opportunity.

At operation 715, the target RAN node 320 signals the assigned downlinkresources in a DCI message encoded with the RMG RNTI received from thesource RAN node 330 in operation 712.

At operation 716, using the assigned downlink resources, the UE 310receives (the first segment of) an RLC data PDU containing the encryptedPDCP data PDU and corresponding logical data channel identifier.

At operation 717, when it successfully decodes the downlinktransmission, the UE 310 responds to the current serving (target) RANnode 320 with an uplink transmission comprising a positiveacknowledgement (e.g. using a HARQ and/or an RLC ACK) with the uplinktransmission advanced according to the timing adjustment computed inoperation 704.

At operation 718, using the security configuration associated with thelogical data channel identifier, the UE 310 decrypts the PDCP data PDUand processes the resulting user plane data PDU.

FIG. 8 illustrates, in one embodiment, a scheme of operation in deliveryof downlink data via RMG flooding. If the UE 310 is not configured forserving cell tracking while operating in an autonomous mode, the currentlocation of the UE may not be known. Since the UE 310 will besynchronised with the serving cell, downlink transmissions can bedirectly scheduled without incurring overheads and latencies associatedwith paging and a subsequent page response using a random accessprocedure.

To avoid latencies and overheads associated with paging to determine thecurrent location of the UE 310, delivery of downlink data to the UE 310involves forwarding of the downlink data from the source RAN node 330 toall target RAN nodes 360 in the RMG 300.

At operation 801, as in operation 501 of FIG. 5 , the configuration foran RMG is distributed to all RAN nodes with cells in the target RMG.

At operation 802, as in operation 502 of FIG. 5 , the UE 310 is preparedfor autonomous operation within the RMG. In addition, the source RANnode 330 may configure the UE 310 with a discontinuous reception (DRX)schedule for use by the UE 310 within the RMG, indicating when the UE310 should monitor the physical downlink control channel (PDCCH) of itsserving cell for notifications of pending downlink data delivery.

At operation 803, once configuration by the source RAN node 330 has beencompleted, the UE 310 may begin autonomous operation within the RMG.

At operation 804, if the UE 310 determines that the downlink signalquality in its current serving cell has dropped below an acceptablethreshold, the UE 310 autonomously selects a new target serving cell asdescribed in operation 504 and derives an uplink transmission timingadjustment for that cell (TA_t) as described in operations 505 to 508 ofFIG. 5 .

At operation 805, at some point in time, the source RAN node 330receives a downlink user data PDU destined for the UE 310 from the CNUPF 345.

At operation 806, using the security configuration associated with theUE context, the source RAN node 330 constructs and encrypts a PDCP dataPDU containing the downlink user data PDU.

At operation 807, since the current location of the UE 310 is unknown,the source RAN node 330 forwards the PDCP data PDU to all of the targetRAN nodes 360 in the RMG along with the RMG RNTI and the DRXconfiguration provided to the UE in operation 802. The source RAN node330 may also include an indication of the type of delivery status reportthat the target RAN node 320 should return to the source RAN node.

At operation 808, each of the target RAN nodes 360 uses the DRXconfiguration to schedule a transmission to the UE 310 at the nextdownlink transmission opportunity.

At operation 809, similarly, UE 310 uses the DRX configuration providedin operation 802 to monitor the downlink control channel (e.g. PDCCH) inthe current serving cell at the next downlink transmission opportunity.

At operation 810, each of the target RAN nodes 360 signals the assigneddownlink resources in a DCI message encoded with the RMG RNTI receivedfrom the source RAN node 330 in operation 806.

At operation 811, using the assigned downlink resources in the currentserving cell, the UE 310 receives (the first segment of) an RLC data PDUcontaining the encrypted PDCP data PDU and corresponding logical datachannel identifier.

At operation 812, when the UE 310 successfully decodes the downlinktransmission, the UE 310 responds to the current serving (target) RANnode 320 with an uplink transmission comprising a positiveacknowledgement (e.g. using a HARQ and/or an RLC ACK) with the uplinktransmission advanced according to the timing adjustment computed inoperation 804. Other target RAN nodes 360 that are not serving the UE310 will not receive a positive acknowledgement.

At operation 813, using the security configuration associated with thelogical data channel identifier, the UE 310 decrypts the PDCP data PDUand processes the resulting user plane data PDU.

At operation 814, the target RAN nodes 360 report delivery status to thesource RAN node 330 according to whether they received a positiveacknowledgement to their downlink transmission: if requested by thesource RAN node 330, the serving target RAN node 330 may return anindication of successful delivery and may also return the identity ofthe current serving cell for serving cell tracking; if requested by thesource RAN node, the other target RAN nodes may return an indication ofunsuccessful delivery.

FIG. 9 illustrates, in one embodiment, a scheme of autonomous handoverbetween unsynchronised network cells that circumvents the latenciesassociated with a random access procedure in the new serving cell.

At operation 901, based on measurements received from the UE 310, thesource RAN node 330 selects a new cell (the target cell) to serve theUE. If the target cell is controlled by another RAN node, the source RANnode sends a handover preparation request to the target RAN node 320that may include the identity of the target cell, an identifier assignedto the UE for use in the current serving cell (e.g. cell RNTI cRNTI-1),and the radio link and security configuration used by the UE in thecurrent serving cell.

At operation 902, following admission control, the target RAN node 320provides the source RAN node 330 with configuration information relatedto UE operation in the target cell. This may include an identifierassigned to the UE for use in the target cell (e.g. cell RNTI cRNTI-2),the radio link and security configuration for use by the UE 310 in thetarget cell, and an uplink grant indicating the radio resources to beused by the UE 310 for transmission of an initial uplink message in thetarget cell.

At operation 903, the source RAN node 330 provides the UE 310 with thetarget cell configuration information that includes an identity of thetarget cell and instructs the UE 310 to handover to the target cell.

At operation 904, the UE 310 obtains downlink frame synchronisation withthe target cell by acquiring synchronisation signals broadcast by thecell. In 3GPP LTE and NR, for example, this may be the primary andsecondary synchronisation signals broadcast in a synchronisation signalblock (SSB).

Operation 905, where the UE 310 obtains a precise time reference fromthe source cell (RT_s), may be performed at any time prior to operation907; operation 905 may, for example, be performed prior to operation 904or operation 903.

At operations 906 and 907, the UE 310 derives an uplink timingadjustment. At operation 906, the UE 310 obtains a precise timereference from the target cell (RT_t) and, if it has not already done so(in operation 905), from the source cell (RT_s). In 3GPP LTE and NR, forexample, the precise time reference may be broadcast in a systeminformation block (SIB) or may be provided on-demand to a UE through aradio resource control (RRC) message.

At operation 907, using knowledge of the timing adjustment used in thesource cell (TA_s), the UE 310 computes the timing adjustment requiredin the target cell (TA_t) as described, for example, in operation 406.

At operation 908, using the uplink grant configuration relayed by thesource RAN node 330 in operation 903, the UE 310 waits for the assignedtransmission opportunity.

At operation 909, at the designated transmission opportunity, the UE 310initiates an uplink transmission using the assigned uplink grantresources and with the uplink transmission advanced according to thetiming adjustment computed in operation 907. The information included inthe uplink transmission comprises at least the identifier assigned tothe UE 310 for use in the target cell (e.g. cRNTI-2).

At operation 910, the target RAN node 320 decodes the uplinktransmission, obtains the identifier (cRNTI-2) transmitted by the UE310, and retrieves the associated UE context.

At operation 911, the target RAN node 320 identifies the source RAN node330 associated with the UE and sends an indication to the source RANnode 330 that handover of the UE 310 is complete, allowing the sourceRAN node 330 to delete the UE context that it has stored.

It is noted that in an embodiment where the target cell is alsocontrolled by the source RAN node 330, the autonomous handover procedureonly includes operations 903 through 909.

When a UE is located in a new serving (target) RAN node throughhandover, through initiation of an uplink transmission, through aserving cell tracking report, or through an RMG delivery status report,the last serving (source) RAN node may choose either to retain its roleas the source RAN node for UE autonomous operations or to transfer therole of source node to the new serving RAN node.

If the last serving RAN node chooses to retain its role as the sourceRAN node, it may signal its intent to the new serving RAN node but nofurther action may be required.

If the last serving RAN node chooses to transfer the role of source nodeto the new serving RAN node, operations at the last serving RAN node mayinclude transferring the UE context to the new serving RAN node, andindicating to the other target RAN nodes in the RMG that it is no longerassociated with the RMG RNTI thereby disabling autonomous operationwithin the RMG for that RNTI.

If the last serving RAN node chooses to transfer the role of source nodeto the new serving RAN node, operations at the new serving RAN node mayinclude sending a path switch request to the CN to ensure thatsubsequent downlink PDUs are delivered to the new serving RAN node,reconfiguring the UE to either disable autonomous operation or tore-enable autonomous operation with a possibly different RMGconfiguration, and configuring a possibly new RMG and the associated setof target RAN nodes if autonomous operation is re-enabled for the UE.

FIG. 10 illustrates a further structure of an electronic device (ED)1000 that includes, in one embodiment, a processor 1005, a networkcommunication interface 1010 and a non-transitory memory 1015. In someembodiments, the UE, such as UE 310, is an ED 1000 that can beconfigured to perform the operations as described herein. In someembodiments, rather than or in addition to using a general-purposecomputer processor, the ED 1000 can use electronic digital and/or analogcircuitry that is configured to perform the operations as describedherein. The circuitry can include, for example, ASICs, FPGAs, digitallogic gates, etc. In some embodiments, the source RAN node 330 and thetarget RAN node 320 are each an ED 1000 configured to perform theoperations of the source RAN node 330 and the target RAN node 320 asdescribed herein.

The ED 1000 may also include a clock 1017 which can be synchronized toprecise time references of the network, for example by incorporatingtiming adjustments. For example, timing adjustments may be added to thecurrent clock time when the clock is updated to reflect a receivedprecise time reference.

FIG. 11 illustrates, in one embodiment, a method 1100 of operationperformed in a user equipment (UE), such as UE 310, for autonomoushandover to a target cell of a radio access network (RAN).

The method includes, at operation 1110, receiving, by the UE, aconfiguration for autonomous operation within a group of cells of theRAN.

The method includes, at operation 1120, determining, by the UE, thetarget cell from the group of cells.

The method includes, at operation 1130, deriving, by the UE, an uplinktiming adjustment that establishes uplink communication synchronizationwith the target cell.

The method includes, at operation 1140, transmitting, by the UE, aprotocol data unit (PDU) to the target cell in accordance with thederived uplink timing adjustment.

FIG. 12 illustrates, in one embodiment, a method 1200 of operationperformed in a user equipment (UE), such as UE 310, for autonomoussynchronisation within a radio access network (RAN).

The method includes, at operation 1210, receiving, at the UE, a firstprecise time reference from a first cell of the RAN.

The method includes, at operation 1220, determining a first uplinktiming adjustment for use with the first cell, the first uplink timingadjustment for establishing uplink synchronization with the first cell.The first uplink timing adjustment may be determined based at least inpart on a timing adjustment message transmitted to the UE from the firstcell. Alternatively, the first uplink timing adjustment may be derivedby the UE in a similar manner to the second uplink timing adjustment, asdescribed herein. This may be the case when the UE has been previouslymoving from cell to cell. However, in this case, it may be required toobtain an original timing adjustment (on which to base subsequent timingadjustments, including the first and second timing adjustment) based ona timing adjustment message transmitted to the UE from some priorutilized cell.

The method includes, at operation 1230, receiving, at the UE, a secondprecise time reference from a second cell of the RAN. It is noted thatthe first and second precise time references can be receivedconcurrently or sequentially. For example, the first precise timereference can be obtained during UE usage of the first cell, and thenheld by the UE for a validity period, which may depend on the precisionof the UE's clock and hence its capability to keep precise time.

The method includes, at operation 1240, deriving, by the UE, a seconduplink timing adjustment that establishes uplink synchronization withthe second cell, the second uplink timing adjustment derived from thefirst and second precise time references and the first uplink timingadjustment.

FIG. 13 illustrates, in one embodiment, a method 1300 performed by auser equipment (UE), such as UE 310, for autonomous handover from asource cell to a target cell of a radio access network (RAN). The method1300 may be carried out by software executed, for example, by theprocessor 1005 of the UE 310 (i.e., when the UE 310 is an ED 1000). Thesoftware may include machine readable instructions which are executableby the processor 1005 to perform the method 1300. The machine readableinstructions of the software may be stored in a non-transitory machinereadable medium, such as the memory 1015 of the UE 310. Coding ofsoftware for carrying out the method 1300 is within the scope of aperson of ordinary skill in the art provided the present disclosure. Themethod 1300 may contain additional or fewer operations than shown and/ordescribed, and may be performed in a different order.

The method 1300 begins at operation 1310. At operation 1310, the UE 310determine an identity of a target cell of a RAN. In some embodiments,the identity of the target cell is determined by the UE based ondownlink signal measurements performed by the UE 310. In someembodiments, the downlink signal measurements includes a downlink signalquality measurement. After the identity of the target cell is determinedat operation 1310, the method 1300 proceeds to operation 1320. Atoperation 1320, the UE 310 receives a precise time reference broadcastby a target cell in the RAN that indicates an elapsed time from apre-determined epoch. In some embodiments, the precise time reference isbroadcast by the target cell in a system information block (SIB).

After the UE 310 receives a precise time reference at operation 1320,the method 1300 proceeds to operation 1330. At operation 1330, the UE310 derives an uplink timing adjustment based on the precise timereference, where the uplink timing adjustment establishing uplinkcommunication synchronization with the target cell of the RAN. In someembodiments, the UE 310 derives uplink timing adjustment further basedon a timing adjustment message transmitted to the UE from the sourcecell. After the UE 310 derives an uplink timing adjustment at operation1330, the method 1300 proceeds to operation 1340. At operation 1340, theUE 310 transmits an uplink signal to the target cell in accordance withthe derived uplink timing adjustment, and the method 1300 ends.

In some embodiments, after the UE 310 transmits an uplink signal to thetarget cell at operation 1340, the method 1300 proceeds to operation1350. At operation 1350, the UE 310 determines that the downlink signalquality measurement has dropped below a pre-established signal qualitythreshold and the UE 310 selects a new target cell of the RAN. After theUE 310 selects a new target cell at operation 1350, the method 1300resumes at operation 1320.

FIG. 14 illustrates, in one embodiment, a method 1440 performed by auser equipment (UE), such as UE 310, for autonomous synchronisationwithin a radio access network (RAN). The method 1400 may be carried outby software executed, for example, by the processor 1005 of the UE 310(i.e., when the UE 310 is an ED 1000). The software may include machinereadable instructions which are executable by the processor 1005 toperform the method 1400. The machine readable instructions of thesoftware may be stored in a non-transitory machine readable medium, suchas the memory 1015 of the UE 310. Coding of software for carrying outthe method 1400 is within the scope of a person of ordinary skill in theart provided the present disclosure. The method 1400 may containadditional or fewer operations than shown and/or described, and may beperformed in a different order.

The method 1400 begins at operation 1410. At operation 1410, the UE 310receives a first precise time reference from a first cell of the RANthat indicates a first elapsed time from a predetermined epoch. Afterthe UE 310 receives a first precise time reference at operation 1410,the method 1400 proceeds to operation 1420. In some embodiments, thefirst precise time reference is received in a broadcast in a systeminformation block (SIB) broadcast, or a radio resource control (RRC)message. At operation 1420, the UE 310 determines a first uplink timingadjustment for use with the first cell to establish uplinksynchronization with the first cell. In some embodiments, the firstuplink timing adjustment is determined by the UE 310 based on a timingadjustment message transmitted to the UE 310 from the first cell. Afterthe UE 310 determines the first uplink timing adjustment at operation1420, the method 1400 proceeds to operation 1430. At operation 1430, theUE 310 receives a second precise timing reference from the target cellof the RAN based on a second elapsed time form the predetermined epoch.In some embodiments, the second precise timing reference is received inan SIB broadcast. After the UE 310 receives a second precise timingreference at operation 1430, the method 1400 proceeds to operation 1440.At operation 1440, the UE 310 derives a second uplink timing adjustmentthat establishes uplink synchronization with the target cell of the RAN.After the UE 310 derives a second uplink timing adjustment, the method1400 ends.

In some embodiments, the UE 310 derives the second uplink timingadjustment by synchronizing a local clock of the UE to the first precisetime reference using the first uplink timing adjustment, and bydetermining the second uplink timing adjustment based on the differencebetween the local clock and the second precise time reference.

In some embodiments, after the UE 310 derives a second uplink timingadjustment, the method 1400 proceeds to operation 1450. At operation1450, the UE 310 performs uplink data transmission to the target cellwith the uplink data transmission advanced according to the seconduplink timing adjustment. After the UE 310 performs the uplink datatransmission, the method 1400 ends. In some embodiments, the uplink datatransmission includes an identifier assigned to the UE 310 for use inthe target cell of the RAN. The identifier assigned to the UE may beused to identify the UE in subsequent messages transmitted by the targetcell. In some embodiments, the subsequent messages include downlinkcontrol information (DCI) transmitted over a physical downlink controlchannel (PDCCH).

The above-described operations may be performed by one or morefunctional modules of a UE 310. As mentioned above, the UE 310 may be anED 1000, which includes at least a processor 1005, network interface1010 and memory 1015, operating in concert to perform any of theabove-described operations. The functional modules may also performother operations, such as conventional transmission and receptionoperations. The modules may be enabled by a processor 1005 executingprogram instructions stored in memory 1015, by particular electronicdigital and/or analog circuitry, or a combination thereof. For example,referring again to FIG. 10 , the ED 1000 may include an autonomous celltargeter 1020 configured to determine a target cell with which tocommunicate, and a timing adjuster 1030 configured to derive uplinktiming adjustments for communication with the target cell. The timingadjuster 1030 can be configured to receive precise time references,obtain prior stored timing adjustments for previously accessed cells,and derive new timing adjustments for accessing new target cells.

Through the descriptions of the preceding embodiments, the presentinvention may be implemented by using hardware only or by using softwareand a necessary universal hardware platform. Based on suchunderstandings, the technical solution of the present invention may beembodied in the form of a software product. The software product may bestored in a non-volatile or non-transitory storage medium, which can bea compact disk read-only memory (CD-ROM), USB flash disk, or a removablehard disk. The software product includes a number of instructions thatenable a computer device (personal computer, server, or network device)to execute the methods provided in the embodiments of the presentinvention. For example, such an execution may correspond to a simulationof the logical operations as described herein. The software product mayadditionally or alternatively include number of instructions that enablea computer device to execute operations for configuring or programming adigital logic apparatus in accordance with embodiments of the presentinvention.

Although the present invention has been described with reference tospecific features and embodiments thereof, it is evident that variousmodifications and combinations can be made thereto without departingfrom the invention. The specification and drawings are, accordingly, tobe regarded simply as an illustration of the invention as defined by theappended claims, and are contemplated to cover any and allmodifications, variations, combinations or equivalents that fall withinthe scope of the present invention.

What is claimed is:
 1. A method for autonomous time synchronizedhandover from a source cell of a radio access network (RAN) to a targetcell of the RAN, the method comprising: selecting, by a user equipment(UE), the target cell of the RAN for receiving an uplink transmissionfrom the UE; receiving, by the UE, a precise time reference broadcast bythe target cell, the precise time reference indicating an elapsed timefrom a predetermined epoch; deriving, by the UE, an uplink timingadjustment based at least in part on the precise time reference, theuplink timing adjustment establishing uplink communicationsynchronization with the target cell; and transmitting, by the UE, theuplink transmission in accordance with the derived uplink timingadjustment.
 2. The method of claim 1 wherein the uplink timingadjustment is further derived based at least in part on reception, bythe UE, of the timing adjustment message from the source cell.
 3. Themethod of claim 1 wherein deriving, by the UE, the uplink timingadjustment is further based at least in part on reception, by the UE, ofa second precise time reference transmitted by the source cell, thesecond precise time reference indicating a second elapsed time from thepredetermined epoch.
 4. The method of claim 3 wherein the second precisetime reference is received in at least one of a system information block(SIB) or a radio resource control (RRC) message transmitted by thesource cell.
 5. The method of claim 3 wherein transmitting, by the UE,the uplink transmission occurs within a pre-determined period of timefollowing reception, by the UE, of the second precise time referencetransmitted by the source cell.
 6. The method of claim 1 whereinselecting, by the UE, further comprises selecting the target cell basedon a downlink signal quality measurement associated with the targetcell.
 7. The method of claim 1 wherein selecting, by the UE, furthercomprises selecting the target cell based at least in part ondetermining that a downlink signal quality measurement associated withthe source cell has dropped below a pre-established signal qualitythreshold.
 8. The method of claim 1 wherein the precise time referenceis received in a system information block (SIB) broadcast by the targetcell.
 9. The method of claim 1 wherein the uplink transmission includesan identifier associated with the UE.
 10. The method of claim 1 whereinthe selecting further comprises selecting the target cell based at leastin part on determining, by the UE, that a pre-determined period of timehas elapsed.
 11. The method of claim 1 wherein the selecting furthercomprises selecting the target cell based at least in part on reception,by the UE, of a message from the source cell, the message from thesource cell including an identifier associated with the target cell. 12.A user equipment (UE) comprising: one or more processors; a radiointerface; and a non-transitory memory storing instructions which, whenexecuted by the one or more processors cause the one or more processorsto: select a target cell of a radio access network (RAN) for receivingan uplink transmission; receive, using the radio interface, a precisetime reference broadcast by the target cell, the precise time referenceindicating an elapsed time from a predetermined epoch; derive an uplinktiming adjustment based at least in part on the precise time reference,the uplink timing adjustment establishing uplink communicationsynchronization with the target cell; and transmit, using the radiointerface, the uplink transmission to the target cell in accordance withthe derived uplink timing adjustment.
 13. The UE of claim 12 wherein theinstructions, when executed by the one or more processors, further causethe one or more processors to receive, using the radio interface, atiming adjustment message from a source cell of the RAN and wherein theuplink timing adjustment is further derived based at least in part onthe timing adjustment message from the source cell.
 14. The UE of claim12 wherein the instructions, when executed by the one or moreprocessors, further cause the one or more processors to receive, usingthe radio interface, a second precise time reference transmitted by asource cell of the RAN wherein deriving the uplink timing adjustment isfurther based at least in part on the second precise time reference fromthe source cell, the second precise time reference indicating a secondelapsed time from a predetermined epoch.
 15. The UE of claim 13 whereinthe uplink transmission is transmitted within a pre-determined period oftime following reception, using the radio interface, of the secondprecise time reference transmitted by the source cell.
 16. The UE ofclaim 14 wherein the instructions, when executed by the one or moreprocessors, further cause the one or more processors to transmit, usingthe radio interface, the uplink transmission within a pre-determinedperiod of time following reception of the second precise time referencetransmitted by the source cell.
 17. The UE of claim 12 wherein theinstructions, when executed by the one or more processors, further causethe one or more processors to measure the quality of a downlink signalreceived, using the radio interface, from the target cell whereinselecting the target cell of the RAN is based, at least in part, on thedownlink signal quality associated with the target cell.
 18. The UE ofclaim 12 wherein the instructions, when executed by the one or moreprocessors, further cause the one or more processors to measure thequality of a downlink signal received, using the radio interface, from asource cell of the RAN wherein selecting the target cell of the RAN isbased, at least in part, on the downlink signal quality associated withthe source cell dropping below a pre-established signal qualitythreshold.
 19. The UE of claim 12 wherein the precise time reference isreceived in a system information block (SIB) broadcast by the targetcell.
 20. The UE of claim 12 wherein the uplink transmission includes anidentifier associated with the UE.